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We have an old mercury switch thermostat in our house, You know the type, gold ring, very common.

We are in new England on a gas/steam system. There is no central AC.

We have been told by more than one heating professional that there really aren't any decent alternatives. The problem is that the thermostat doesn't report the right temperature, so it basically functions as a switch. The other problem is that my wife hates it.

I think we are on a 2 rather than three wire system. The house was built in the late 1920's.

Does anyone have experience with alternative thermostats?

mmcglynn
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    Why do the professionals say there aren't any good alternatives? There are countless other thermostats on the market now. – BMitch Nov 04 '13 at 13:11
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    replacing mine like that for a moderately priced digital was the best thing I ever did. It seemed that for A/C, 60 degrees and 80 degrees were about 1 degree of rotation apart. :) Now I just need to get it recycled properly… It absolutely should not go into the trash. – Tim B Nov 04 '13 at 13:42
  • see [Replace two-wire thermostat](http://diy.stackexchange.com/questions/6161/replace-two-wire-thermostat?rq=1) – mac Nov 04 '13 at 14:53

6 Answers6

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Almost every thermostat on the market today is a better alternative than a mercury switch-type.

Even the cheapest digital thermostats have schedules, so that they can have different heating points during the morning, daytime, evening and overnight, and range from having weekend vs weekday to 7-day indepedent schedules. This allows you to save money by turning down the temperature while you're not home or sleeping.

typical digital thermostat

On the more advanced side of things, there are internet-connected thermostats you can program/control from your PC/phone/etc, and ones that can self-learn based on manual adjustments and motion-sensing. There is a corresponding increase in price, and these typically do require a C-wire so since you only have two wires you'd have to pull a new wire for this.

NEST thermostat

To connect a basic digital thermostat to the 2-wires, the best thing is to follow the instructions that come with the thermostat. Typically you're going to connect the two wires between R (or Rh) and W.

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gregmac
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    Agree that the modern thermostats are a huge improvement, but consider: a bi-metallic strip and a glass bulb with two electrodes and a drop of mercury. Functions for close to a century. No firmware upgrades, no Blue Screen of Death, no re-booting... – DJohnM Nov 04 '13 at 19:08
  • I recently had the first mercury t stat failure I can remember at my house Built in 1930. Mine was probably a late 50' S early 60' S not original. There are modern looking ones out There and I could have gone with a programmable but went with the simple 9$ no batteries needed stat. – Ed Beal Jan 15 '18 at 16:28
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I agree with John (?) - simple, reliable, doesn't require batteries or power. Indeed, if you have gas you can have normal heat during power outages. If you are frustrated with the lack of temperature sensitivity, you might try messing with the bi - metallic strip and change its performance characteristics.

Sab
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I have a mercury thermostat and I have been told by more than one heating and cooling guy that it's the best. It's simple and doesn't freeze up. Ours isn't the most accurate but it does the job.

Andrew
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Mercury thermostats are clearly the best for many reasons not the least of which is they will last forever and never need a battery. They also have fully adjustable heat anticipators which NONE of the digital or Nest thermostats have. ....... Digital and Nest thermostats have dozens of components to fail and die without battery power. So they simply turn what WAS a reliable furnace into a Rube Goldberg machine that breaks down every heating season.

Jim Navotney
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  • I can see that point. For the record I have not replaced the thermostat, mostly because I ask every plumber I know and they say don't do it, maybe for the reasons your state. – mmcglynn Nov 30 '18 at 20:58
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Interesting post. I have mercury switch thermostats. Had the first problem with it ever. It was at the beginning of the heating season. I had set the themostat all the way down for the summer. But the mercury got "stuck" at the end of the glass tube and I had an apparent no heat situation. I took the cover off the thermostat, saw the problem which was easily fixed by turning the thermostat all the way up to the highest heat setting. I watched the mercury roll out of the tip of the tube where it had been "stuck" and turn on the heat. After that it worked fine as it has for years. Requires no internet or battery.

Mercury switch Bob
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For 30 years we had an old round mercury thermostat. It worked great. Then we bought a new Trane furnace with a new digital honeywell stat like the one above. The new stat freezes. It's programed and constantly gets stuck on the previous setting and doesn't adjust when it's supposed to unless I press a button, any button, then it wakes up and almost like it says "oh, sorry, I was sleeping" and corrects it's self. Im tempted to reinstall the mercury stat. And this isn't the 1st digital stat I've had problems with. It seems the digital stats I've encountered have a short lifespan, like smart phones. Perhaps they're designed not to last more than 2-3 years.

James
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  • Congradulations, you are the one millionth customer to complain about a honeywell thermostat. Don't bother asking, the HVAC guy checked and couldn't find any problem. – Ben Welborn Apr 06 '16 at 21:06